Translucent Object Rendering Methods and Apparatus

ABSTRACT

A computer system method includes determining a first surface illumination on an object based upon a respective irradiance, determining a first surface reflection based upon the respective irradiance, determining a second surface illumination on the object based upon a respective irradiance, determining a second surface reflection based upon the respective irradiance, receiving a first weighting for the first illumination and the first reflection value, and a second weighting for the second illumination and the second reflection value, wherein the weightings are different, determining a first surface illumination in response to the first illumination value, the first reflection, and the first weighting, determining a second surface illumination in response to the second illumination value, the second reflection, and the second weighting, and determining an image including the object based on the first surface illumination and the second surface illumination.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to and is a non-provisionalapplication of U.S. Provisional Patent Ser. No. 61/060,026 filed Jun. 9,2008. This application is also a continuation-in-part of 11/084,697,filed Mar. 18, 2005, now allowed. This case is also related toapplication Ser. No. ______, filed ______, titled “Subsurface RenderingMethods and Apparatus, attorney docket no. 021751-000492US. Thesedisclosures are hereby incorporated by reference in their entirety forall purposes.

BACKGROUND OF THE INVENTION

The present invention relates to computer animation. More particularly,the present invention relates to techniques and apparatus for efficientrendering of objects that are specified to include translucentmaterials.

Throughout the years, movie makers have often tried to tell storiesinvolving make-believe creatures, far away places, and fantastic things.To do so, they have often relied on animation techniques to bring themake-believe to “life.” Two of the major paths in animation havetraditionally included, drawing-based animation techniques and physicalanimation techniques.

Drawing-based animation techniques were refined in the twentiethcentury, by movie makers such as Walt Disney and used in movies such as“Snow White and the Seven Dwarfs” (1937) and “Fantasia” (1940). Thisanimation technique typically required artists to hand-draw (or paint)animated images onto a transparent media or cells. After painting, eachcell would then be captured or recorded onto film as one or more framesin a movie.

Physical-based animation techniques typically required the constructionof miniature sets, props, and characters. The filmmakers would constructthe sets, add props, and position the miniature characters in a pose.After the animator was happy with how everything was arranged, one ormore frames of film would be taken of that specific arrangement.Physical animation techniques were developed by movie makers such asWillis O'Brien for movies such as “King Kong” (1933). Subsequently,these techniques were refined by animators such as Ray Harryhausen formovies including “Mighty Joe Young” (1948) and Clash Of The Titans(1981).

With the wide-spread availability of computers in the later part of thetwentieth century, animators began to rely upon computers to assist inthe animation process. This included using computers to facilitatedrawing-based animation, for example, by painting images, by generatingin-between images (“tweening”), and the like. This also included usingcomputers to augment physical animation techniques. For example,physical models could be represented by virtual models in computermemory, and manipulated.

One of the pioneering companies in the computer animation (CA) industrywas Pixar. Pixar is more widely known as Pixar Animation Studios, thecreators of animated features such as “Toy Story” (1995) and “Toy Story2” (1999), “A Bugs Life” (1998), “Monsters, Inc.” (2001), “Finding Nemo”(2003), “The Incredibles” (2004), “Cars” (2006), “Ratatouille” (2007),“Wall-E” (2008) and others. In addition to creating animated features,Pixar developed computing platforms specially designed for CA, and CAsoftware now known as RenderMan®. RenderMan® was particularly wellreceived in the animation industry and recognized with two AcademyAwards®.

RenderMan® software is used to convert graphical specifications ofobjects and convert them into one or more images. This technique isknown in the industry as rendering. One specific portion of therendering process is known as surface shading. In the surface shadingprocess the software determines how much light is directed towards theviewer from the surface of objects in an image in response to theapplied light sources.

For different types of object materials, however, the amount of lightdirected towards the viewer is more than the light reflected from thesurface. Such types of materials include materials that are at leastpartially translucent in nature, such as plastic, skin, milk, and thelike. Translucent materials may also include materials that arenon-uniformly translucent, such as marble, jade, and the like, and mayhave sub-surface features. Compensation for subsurface scattering andother effects of translucency are deemed important for the realism ofcomputer-generated imagery.

Techniques to compensate and to render translucent objects havepreviously been proposed and generally fall into two categories: surfacebased approaches, and stochastic ray tracing methods. Each of theseapproaches has significant shortcomings. Surface based approaches sufferfrom technical difficulties in the parametric representation of samplelocations. Additionally, such approaches do not address the fundamentalproblem of representing an internal structure of the scattering media.

Stochastic ray tracing approaches can be used to solve the full physicalproblem, however such approaches are generally very time consuming.Another approach includes a finite element analysis through ahierarchical evaluation of a radiative transfer equation. Again suchapproaches are very computationally intensive. Because the presentsolutions to the problem of rendering objects of translucent materialsis slow, animators using such rendering techniques cannot quickly seehow the animation will look. Accordingly, the animators must often waitovernight to see the rendering results.

In light of the above, what is needed are efficient techniques to rendertranslucent objects.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to the field of computer animation. Moreparticularly, the present invention relates to techniques and apparatusfor efficient rendering of objects composed of translucent materials.

The techniques described below are based on an explicit representationof the subsurface illumination fluxes on a 3 dimensional grid. Thisrepresentation allows the efficient spatial indexing of illuminationsamples from the unstructured geometry of the object. Thisrepresentation allows the rendering engine to quickly solve thetransport of diffuse illumination inside the translucent material withfull representation of the internal structure.

The process described below includes the following phases: 1) evaluationof surface illumination on object, collection of surface illumination(diffuse reflection), determining of area weighting, and scatteringproperties on sample grids; 2) efficient determination of the diffuseradiative transfer on the sample grids; and 3) sampling the diffuseillumination during final rendering pass.

In this embodiment, the first phase is the illumination sampling pass.In this phase, the relevant light sources are evaluated on the surfaceof an object and the surface shading model is evaluated to determine thequantity of diffuse illumination (diffuse reflection) for each colorchannel. The diffuse illumination (diffuse reflection) at the surfaceare transmitted to the translucency solver, described below.Additionally, Cartesian solver grids are constructed which cover thebounding volume of the object sample points. The diffuseillumination/diffuse reflection values are inserted onto the solvergrids where they are merged with other values. The merge process isaccomplished by adding the representative sample area for the sampleinto one grid and simultaneously adding the product of the sample areaand the illumination value are added into a second grid. Scatteringproperties such as attenuation and scattering length are also mappedonto grid structures using the same technique. For internal structureswith varying scattering properties additional samples of theseproperties are added. During this phase the samples may be offset fromthe actual sample position using the surface normal to simulate theeffects of an initial forward or an initial reverse scattering processbefore the illumination enters the diffuse solver.

The second phase of the process is the solution of the scattering passitself. In this embodiment, the illumination and scattering propertiesare normalized by processing all of the grids with a short rangedealiasing filter and diving the area weighted illumination andscattering property grids by the area grid value. The diffuseillumination compensation values are then solved by solving the ellipticradiative transfer equation (1) on the grid values. This process can bedone efficiently in several ways. For example, in one approach theprocess is performed via a recursive low pass filter whose filtercoefficients vary with scattering parameters across the grid in eachdirection for each dimension. The filter parameters are chosen so thatthe bidirectional filter impulse response approximates the Green'sfunction of the scattering operator with the local scatteringparameters. In another approach, the variable coefficient ellipticscattering equation is solved using a fast elliptic solver such as afast convolution FFT solver or multi-grid solver.

The third phase of the process is to take the completed grid containingthe solved diffuse illumination compensation values and point sample thegrid using an interpolation kernel from the surface shaders. This mayoccur during a final rendering pass for the surface. All of the sampleprocessing for this phase is done in global 3-space so that no surfacecoordinate parameterizations are needed and smoothness across multiplegeometry elements is provided.

According to various embodiments, a method for a computer system isdescribed. One technique includes determining a first diffuseillumination compensation value associated with a first surface regionon an object in response to an irradiance associated with the firstsurface region, determining a first diffuse reflection value associatedwith the first surface region on the object in response to theirradiance associated with the first surface region, determining asecond diffuse illumination compensation value associated with a secondsurface region on the object in response to an irradiance associatedwith the second surface region, and determining a second diffusereflection value associated with the second surface region on the objectin response to the irradiance associated with the second surface region.A process may include receiving a specification of a first weighting forthe first diffuse illumination compensation value and the first diffusereflection value, and a second weighting for the second diffuseillumination compensation value and the second diffuse reflection value,wherein the first weighting is different from the second weighting,determining a diffuse illumination value associated with the firstsurface region on the object in response to the first diffuseillumination compensation value, the first diffuse reflection, and thefirst weighting, determining a diffuse illumination value associatedwith the second surface region on the object in response to the seconddiffuse illumination compensation value, the second diffuse reflection,and the second weighting, and determining an image including a renderedrepresentation of the object in response to the diffuse illuminationvalue associated with the first surface region and to the diffuseillumination value associated with the second surface region.

According to various embodiments, a computer system is described. Oneapparatus includes a memory configured to store a model of an objecthaving a first surface region and a second surface region. A device mayinclude a processor coupled to the memory, wherein the processor isconfigured to determine a first diffuse illumination compensation valueassociated with the first surface region in response to an irradianceassociated with the first surface region, wherein the processor isconfigured to determine a first diffuse reflection value associated withthe first surface region in response to the irradiance associated withthe first surface region, wherein the processor is configured todetermine a second diffuse illumination compensation value associatedwith a second surface region in response to an irradiance associatedwith the second surface region, wherein the processor is configured todetermine a second diffuse reflection value associated with the secondsurface region in response to the irradiance associated with the secondsurface region, wherein the processor is configured to receive aspecification of a first weighting for the first diffuse illuminationcompensation value and the first diffuse reflection value, and a secondweighting for the second diffuse illumination compensation value and thesecond diffuse reflection value, wherein the first weighting isdifferent from the second weighting, wherein the processor is configuredto determine a diffuse illumination value associated with the firstsurface region in response to the first diffuse illuminationcompensation value, the first diffuse reflection, and the firstweighting, wherein the processor is configured to determine a diffuseillumination value associated with the second surface region in responseto the second diffuse illumination compensation value, the seconddiffuse reflection, and the second weighting, and wherein the processoris configured to determine an image including a rendered representationof the object in response to the diffuse illumination value associatedwith the first surface region and to the diffuse illumination valueassociated with the first surface region. In various embodiments, thememory is also configured to store the image.

According to various embodiments, a computer program product for acomputer system comprising a processor and a memory, wherein thecomputer program product comprises a computer-readable tangible mediacomprising executable computer code is described. In variousembodiments, the computer program product may include code that directsthe processor to determine a first diffuse illumination compensationvalue associated with a first surface region of an object in response toan irradiance associated with the first surface region, code thatdirects a processor to determine a first diffuse reflection valueassociated with the first surface region in response to the irradianceassociated with the first surface region, code that directs a processorto determine a second diffuse illumination compensation value associatedwith a second surface region of the object in response to an irradianceassociated with the second surface region, and code that directs aprocessor to determine a second diffuse reflection value associated withthe second surface region in response to the irradiance associated withthe second surface region. The computer program product may include codethat directs a processor to receive a specification of a first weightingfor the first diffuse illumination compensation value and the firstdiffuse reflection value, and a second weighting for the second diffuseillumination compensation value and the second diffuse reflection value,wherein the first weighting is different from the second weighting, codethat directs a processor to determine a diffuse illumination valueassociated with the first surface region in response to the firstdiffuse illumination compensation value, the first diffuse reflection,and the first weighting, code that directs a processor to determine adiffuse illumination value associated with the second surface region inresponse to the second diffuse illumination compensation value, thesecond diffuse reflection, and the second weighting, and code thatdirects a processor to determine an image including a renderedrepresentation of the object in response to the diffuse illuminationvalue associated with the first surface region and to the diffuseillumination value associated with the first surface region.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the present invention, reference ismade to the accompanying drawings. Understanding that these drawings arenot to be considered limitations in the scope of the invention, thepresently described embodiments and the presently understood best modeof the invention are described with additional detail through use of theaccompanying drawings in which:

FIG. 1 illustrates a block diagram of a rendering system according toone embodiment of the present invention;

FIGS. 2A-C illustrate a block diagram of a flow process according to anembodiment of the present invention;

FIGS. 3A-B illustrate an example of an embodiment of the presentinvention;

FIGS. 4A and 4B illustrates examples according to embodiments of thepresent invention;

FIGS. 5A and 5B illustrate examples according to embodiments of thepresent invention;

FIG. 6 illustrates a process according to an embodiment of the presentinvention;

FIGS. 7A-B illustrate examples according to embodiments of the presentinvention;

FIGS. 8A-B illustrate examples according to embodiments of the presentinvention;

FIGS. 9A-C illustrate additional examples according to embodiments ofthe present invention;

FIGS. 10A-D illustrate examples according to embodiments of the presentinvention; and

FIG. 11 illustrates an example according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of typical computer system 100 according to anembodiment of the present invention.

In the present embodiment, computer system 100 typically includes amonitor 110, computer 120, a keyboard 130, a user input device 140, anetwork interface 150, and the like.

In the present embodiment, user input device 140 is typically embodiedas a computer mouse, a trackball, a track pad, wireless remote, and thelike. User input device 140 typically allows a user to select objects,icons, text and the like that appear on the monitor 110.

Embodiments of network interface 150 typically include an Ethernet card,a modem (telephone, satellite, cable, ISDN), (asynchronous) digitalsubscriber line (DSL) unit, and the like. Network interface 150 aretypically coupled to a computer network as shown. In other embodiments,network interface 150 may be physically integrated on the motherboard ofcomputer 120, may be a software program, such as soft DSL, or the like.

Computer 120 typically includes familiar computer components such as aprocessor 160, and memory storage devices, such as a random accessmemory (RAM) 170, disk drives 180, and system bus 190 interconnectingthe above components.

In one embodiment, computer 120 is a PC compatible computer havingmultiple microprocessors such as Xeon™ microprocessor from IntelCorporation. Further, in the present embodiment, computer 120 typicallyincludes a UNIX-based operating system.

RAM 170 and disk drive 180 are examples of tangible media for storage ofdata, audio/video files, computer programs, applet interpreters orcompilers, virtual machines, embodiments of the herein describedinvention including geometric scene data, object data files, shaderdescriptors, a rendering engine, output image files, texture maps,displacement maps, scattering lengths and absorption data of objectmaterials, and the like. Other types of tangible media include floppydisks, removable hard disks, optical storage media such as CD-ROMS andbar codes, semiconductor memories such as flash memories,read-only-memories (ROMS), battery-backed volatile memories, networkedstorage devices, and the like.

In the present embodiment, computer system 100 may also include softwarethat enables communications over a network such as the HTTP, TCP/IP,RTP/RTSP protocols, and the like. In alternative embodiments of thepresent invention, other communications software and transfer protocolsmay also be used, for example IPX, UDP or the like.

FIG. 1 is representative of computer rendering systems capable ofembodying the present invention. It will be readily apparent to one ofordinary skill in the art that many other hardware and softwareconfigurations are suitable for use with the present invention. Forexample, the use of other micro processors are contemplated, such asCore™ or Itanium™ microprocessors; Phenom™ or Opteron™ microprocessorsfrom Advanced Micro Devices, Inc; and the like. Further, other types ofoperating systems are contemplated, such as Windows® operating systemsuch as WindowsXP®, WindowsVista®, or the like from MicrosoftCorporation, Solaris™ from Sun Microsystems, LINUX, UNIX, MAC OS X fromApple Incorporated, and the like.

FIGS. 2A-C illustrates a block diagram of a flow process according to anembodiment of the present invention. More particularly, the blockdiagram discloses a process in which the material property of an objectare considered as part of the shading process

In one embodiment of the present invention, the process begins with thespecification of an image to be rendered, step 200. In this example,image specification includes the specification of one or more scenebackgrounds, the specifications of objects and textures in the images,the specification of lighting to be applied to the images, thespecification of object placement data, the specification of animationparameters for objects in the scene, displacement maps, and the like.Typically, images are specified by one or more animators usingconventional three-dimensional modeling techniques and softwarecurrently available. For example, software such as Alias|Wavefront'sMaya™ Software, and the like may be used to specify the scene.

In the present examples, a specification of a background typicallyincludes a geometric specification of a location or a “set” into whichobjects, described below, are placed. The geometric specification mayinclude geometric boundaries, one or more “environment” maps, and thelike. These environment maps may be static or may vary with time. Inexamples of the present invention, a set may represent any locationwhere the objects are placed, such as a room, an interior location, anexterior location, or the like.

The objects are placed or positioned by the animator into the set. Theseobjects may be in inanimate objects within a scene such as furniture,plants, vehicles, and the like. Additionally, these objects may beobjects that move or are moved within the scene, for example,characters, chairs, doors, and the like. In the present embodiment,objects may be simple to complex, with some objects being formed fromone or more smaller objects. In some embodiments, only portions ofobjects may be retrieved from a memory, and the remaining portions ofthe objects are dynamically created before being rendered. In suchcases, the dynamically created objects are termed procedurally generatedobjects.

In the present embodiment, objects are typically stored separately fromeach other. That is, each object may be stored in its own file and beretrieved by a computer file system. In other embodiments of the presentinvention, data for more than one object may be included into a singlefile.

Each scene typically also includes a specification of light sources andlighting parameters. Lighting data may include light sources, lighttypes (e.g. diffuse, point), lighting locations, lighting directions,and the like. This lighting data may be stored within a single file ormultiple files.

Next, an object to be rendered is identified, retrieved from disk memoryand then stored in program memory, step 310. In one embodiment,streaming technology is used so that the rendering engine can beginprocessing the object, although the entire data file has not yet beencompletely retrieved by memory. In other embodiments, the renderingengine begins processing the object after it is retrieved from memory.

In the present embodiment, once the object is retrieved from diskmemory, the surface of the object is shaded, step 320. Any number ofconventional surface shading programs may used for this step, forexample, RenderMan® software may be used to determining the surfaceshading for each light source. In one embodiment, the shading of thesurface is determined for all lighting sources at the same time. In analternative embodiment, the surface of the object is shaded individuallyfor each light source. For example, if there are two light sources,there may be two separate shading values for each location on thesurface of the object.

In this embodiment, a volumetric grid is then constructed to bound theentire object, step 330. In one example, the volumetric grid isCartesian grid with approximately 200 to 500 vertices per side;accordingly the Cartesian grid may include from 8 million to 125 millionvoxels. The number of grid locations per side may be unequal and mayvary, for example, a grid may be constructed to have 110 voxels in an xdirection, 400 in a y direction and 500 in a z direction; thus the gridmay include approximately 22 million voxels. In light of the presentpatent application, one of ordinary skill in the art would understandthat the resolution of the volumetric grid in any direction and totalnumber of voxels defined will vary according to specific engineeringrequirements and resources. In one embodiment, the voxels may be cubes,however in other embodiments, the voxels may have unequal dimensions andbe irregular in shape.

In various embodiments of the present invention, multiple scatteringgrids may be used to bound an object. Use of a single rectangular volumecan be inefficient to bound large and complex objects. For example, forhumanoid characters, a single bounding box would include a large numberof voxels that include empty space. As an example, areas between thelegs, or between the arms and body would typically be empty. These emptyvoxels are defined and maintained in memory, however are not used.Additionally, depending upon how the object is composed ofsub-components, the subsurface scattering effects of the differentsub-components may be effectively independent. For example, diffuseillumination incident on the right hand of an object do not typicallyimpact the subsurface scattering illumination on the left hand.

In various embodiments of the present invention, the multiple grids maybe specific to the pose of the object. As an example, in a first pose anarm of a person is straight, and a single bounding box may be definedfor the arm sub-component. However, in a second pose of the arm, the armis bent at the elbow at 90 degrees. In such a case, if a single boundingbox were used, many empty voxels would be allocated between the hand andthe shoulder. Accordingly, in this example, two or more sub-grids may beallocated, including one sub-grid for the hand and forearm and onesub-grid for the upper arm.

In various embodiments of the present invention, to reduce the amount ofunused voxels computed, objects may first be separated intosub-components and then bounding boxes for the sub-components may becomputed. Voxels in the smaller bounding boxes for the sub-componentsare then more likely to be used for scattering calculations, describedbelow. An example, limbs of a humanoid may be bound within their ownbounding boxes, and a torso may be bound within its own bounding box.According, the empty space between the arms and the body are notallocated. Additionally, such embodiments enable selective rendering ofsub-components of an object. For example, the subsurface scatteringillumination determinations may be performed on the head, but not thetorso or legs for one or more scenes.

One issue that may arise as a result of the use of smaller boundingboxes is the potential of boundary mismatch or inconsistency at theboundaries. In various embodiments of the present invention, to addressthis issue, grids of sub-components are intentionally overlapped betweenboundaries with other sub-components. In various embodiments, thecomputed grid values in the transition area between the sub-componentsare smoothly (e.g. linearly) interpolated, or the like to reduce themismatch.

In various embodiments of the present invention, the inventors havedetermined that often internal geometric elements in a object modelinterfere with sub-surface scattering techniques, such as that describedbelow. More particularly, characters are often modeled with eye socketsor mouth “bag” voids in a model. In operation these internal geometricelements skew the sub-surface scattering computations. This skewingtranslates into unexpected local darkening or glowing of the surface,sometimes known as sparkles and pops.

In various embodiments of the present invention, these voids are ignoredfrom the geometric model thus resulting in a less complex geometricmodel. For example, in one embodiment, a mouth “bag” and eye sockets maybe removed from a head model, thus the geometry of the head model willbecome mostly convex. In the present embodiments, when mapping voxels tothe geometric model, some voxels (“void voxels”) will be associated withregions of the object where the voids were previously located. Inanother embodiment, the voids are tagged and ignored during theirradiance gathering step.

In various embodiments of the present invention, the diffuseillumination compensating values (e.g. subsurface illuminationcontributions), determined below, for these “void voxels” are thensimply discarded and not used for shading purposes. Accordingly, theundesirable artifacts due to these voids are reduced.

In the present example, not all voxels will include a portion of theobject or the surface of the object. For this embodiment, such voxelsare ignored for the following calculations or discarded.

Next, the voxels that bound any portion of the surface of the object(surface voxels) are identified, step 340.

In the present embodiment, the vertices of each surface voxel thatincludes a surface portion of the object are associated with diffuseillumination values. The surface portions typically include shadingsurfaces (e.g. triangles) defined by shading values (diffuseillumination values) and associated shading areas. In this embodiment,the shading values typically include red, green, blue (RGB) shadingcomponents; and the shading areas typically represent an area of arespective shading surface.

In this embodiment, the size of shading areas are smaller than the gridspacing, accordingly, an area weighting operation is performed for eachshading value, step 350.

Next, the area weighting values for each shading value are distributedto each of the 8 vertices in a voxel, step 360. In this embodiment,after each of the area-weighted shading values are distributed to thevertices, the contributions are added-up, step 365. In one embodiment, atypical vertex is associated with contributions of shading surfaces fromdifferent voxels.

FIGS. 3A and 3B illustrate an example of one embodiment of the presentinvention. FIG. 3A illustrates a top view and a profile view of oneexample. In FIG. 3A a number of diffuse illumination values 430 andshading surfaces 440 (including shading surface 420) are illustratedwith solid lines. On top of these values are voxel boundaries 450illustrated with dotted lines (slightly offset up and to the left forclarity). In this embodiment, shading surfaces 440 are triangles,although in other embodiments, the shading surfaces may be triangles ofdifferent geometry and area, or different shapes (e.g. quadrilateral).

In this example, voxels 460, 470 and 480 are illustrated from the topand from the side in profile view in 490. In this example, the diffuseillumination value of shading surface 420 is assumed to be at thecentroid 495 of shading surface 420. Further, as can be seen, centroidis location within voxel 470 ¼ the way from the closest corner 425, ⅕the way in another direction, and ¼ in the third direction. Further, inthis example, the diffuse illumination value is determined to be 40, andthe surface area for shading surface 420 is 5 units.

In this example the area-weighted illumination value is (40×5=200) 200.The area-weighted illumination is then divided in a three to 1 ratio tothe bottom corners. Thus the bottom corners divide 150 and the topcorners divide 50. Next, based upon the position of centroid 495, theseillumination values are further divided in approximately the wayillustrated in FIG. 3B.

In other embodiments of the present invention, any other conventionalmethod for distributing shading values to vertices of voxels iscontemplated. For example, the area-weighted contribution for allshading surfaces may be determined once for the voxel, and that value isdistributed to the eight vertices in the voxel, depending upon position.FIGS. 4A and 4B illustrates such an example.

FIG. 4A illustrates a top view and a profile view of one example. InFIG. 4A, a number of diffuse illumination values 500 and shadingsurfaces 510 are illustrated with solid lines. On top of these valuesare voxel boundaries 520 illustrated with dotted lines (slightly offsetup and to the left for clarity). In this embodiment, shading surfaces510 are squares of equal area, although in other embodiments, theshading surfaces may be quadrilaterals of different area.

In this example, voxels 530, 540 and 550 are illustrated including theshading surfaces and shading values shown. Also illustrated is a profileview 560 illustrating a location of the shading surfaces within voxels530, 540, and 550. In this example, using area weighting calculations,the average illumination value in voxel 530 is illustrated as 32; theaverage illumination value in voxel 540 is 32, and the averageillumination value in voxel 550 is 32. From the top view, the averageillumination values are associated with the centers of the respectivevoxels, i.e. from the top, the average illumination values are assumedto be equidistant to the four respective corners of the voxels. Fromprofile view 560, the average illumination values for voxels 530 and 550associated with the center of the voxels, and are equidistant from thetop and bottom corners, but the average illumination values for voxel540 is associated with a position about 1.7 times closer to the bottomcorners than the top corners.

As illustrated in FIG. 4B, the average diffuse illumination values aredistributed to the respective vertices in voxels 530, 540, and 550 basedupon the positions of the average illumination values, determined above.For voxels 530 and 550, the distribution to the top and bottom verticesis uniform. However, in this example, for voxel 540, because theillumination values are on the average, about 1.7 times closer to thebottom than to the top, the amount of illumination distributed to thebottom vertices (4×5=20) is about 1.7 times the amount of illuminationdistributed to the top vertices (4×3=12).

In the present embodiment, the above process is then typically repeatedto distribute material properties values of the object onto thevertices. In the present embodiment, the material properties includeattenuation factors and scattering lengths for the material bounded byrespective voxels, step 370. Because the attenuation factors andscattering lengths are parameters of the material within the object, andnot just on the surface, the process is performed for all voxels thatinclude a portion of the object. In this embodiment, as a result, thematerial properties are mapped onto vertices of the voxels as avolume-weighted scattering length grid, and a volume-weightedattenuation factoring grid.

At this stage, a number of grids may be allocated and populated, forexample, one grid to store weighted diffuse illumination values, onegrid to store weighted attenuation factors, and one grid to storeweighted scattering lengths. In other embodiments, additional grids maybe allocated to stored additional values. In still another embodiment,only a single grid may be defined, and the weighted diffuse illuminationvalues and weighted material properties are merely associated with thatgrid.

In the present embodiment, the diffuse illumination values, attenuationfactors, and scattering lengths are color dependent. In this example,diffuse illumination values include a red component, a green component,and a blue component, however other color system may also be used. Inone embodiment, the attenuation factors and scattering lengths are colordependent. As an example, for a green jade material, the scatteringlengths of red and blue may be shorter in the material than thescattering length of green in the material. Accordingly, in the presentembodiment, nine volumetric grids may be created and populated. In otherembodiments, a single grid having vertices associated with multiplevalues may also be used.

The following steps are performed to efficiently simulate a scatteringprocess of the translucent material. In the present embodiment, theprocess begins by processing the area-weighted diffuse illumination gridvalues, the volume-weighted scattering length grid values, and thevolume-weighted attenuation factor grid values with a short rangedealiasing filter per color step 380. In various embodiments of thepresent invention, the dealiasing filter may be a blur filter, or thelike.

Next, in this embodiment, the area weighted diffuse illumination values,the volume-weighted scattering lengths, and volume-weighted attenuationfactors are normalized, step 385. In one example, the area weighteddiffuse illumination grid values are divided by a grid storing the areasof the respective voxels; and the volume-weighted scattering length gridvalues and the volume-weighted attenuation factor grid values are bothdivided by a grid storing the volumes of the respective voxels. In otherembodiments, other types of normalizing filters may be used.

Embodiments of the present invention can simulate non-uniform materialswithin the object, such as bones, cartilage, metal, and the like withthe variance across the grid. Very few organic elements are ofhomogeneous nature. Many translucent objects comport internal parts thatocclude the light scattering inside. In the case of skin for instance,the underlying bone and muscle structure plays an important part in thesubsurface scattering. Spatially varying the scatter length across themodel surface however is one way to simulate this internal complexity.To this end, the irradiance gathering process also records a spatiallyvarying tricolor (RGB) scatter length that is being provided by thesurface shader.

In various embodiments of the present invention, the scattering lengthtriad can be generated by any conventional shading methods, such asprocedural texture, painted maps or a combination of both. As examples,a user may retrieve one or more two-dimensional views of athree-dimensional bone, a face, or the like in a two-dimensional paintprogram such as Adobe Photoshop® or the like. Next, the user paints thedesired scattering length values in an overlay layer on top of thetwo-dimensional views. In various embodiments, the color and orintensity of the color is used to signify the desired scattering lengthvalues. For example, the scatter length is painted such that the “depth”of tissue is thinner in a forehead area of a face, and such that the“depth” of tissue is thicker on the cheeks of the face, and the like.

In such embodiments, the painted overlay layer becomes associated withthe two-dimensional view of the bone, the face, or the like. In variousembodiments of the present invention, at a later time, three-dimensionalpaint-projection techniques are subsequently used to map the overlaylayer pixel values back to the three-dimensional object. Featheringtechniques may also be used so that the scattering length valuessmoothly transition about the edges of the bone, face, or the like. Invarious embodiments of the present invention, multiple two-dimensionalviews of a three-dimensional object can be “painted” upon at the sametime, as described in Application No. PCT/US04/16946 filed Mar. 23,2004. That application is incorporated by reference for all purposes.

In various embodiments of the present invention, objects that make-up anobject have scattering-lengths defined, as described above. As anexample, the forearm includes skin and flesh with a first scatteringlength, and underlying bone with a second scattering length. Theserespective scattering lengths are then mapped to vertices of thevolumetric illumination grids, described above.

In various embodiments of the present invention, various methods may beimplemented to facilitate the determination of diffuse illuminationvalues where non-uniform materials are used. In such embodiments,typically more than one layer of material are layered on top of eachother, and may have different attenuations, scattering lengths, or thelike. As merely an example, a skin layer may have a scattering length often units, a muscle layer may have a scattering length of five units,and a cartilage layer may have a scattering length of seven units. Insome embodiments, a uniform scattering length is determined and mappedto vertices of the volumetric illumination grids, based upon therespective thicknesses of materials, and the scattering lengths. Morespecifically, a weighted average may be used for the uniform scatteringlength. Continuing the example above, the skin layer may be one gridunit in thickness, the muscle layer may be two grid units in thickness,and the cartilage layer may be four grid units in thickness.Accordingly, the average scattering length mapped to vertices of thevolumetric illumination grid for these seven grid units in thickness (7grid units=1 grid unit+2 grid units+4 grid units) is 6.8 (6.8 scatteringlength=(10 scattering length*1 grid unit)+(5 scattering length*2 gridunits)+(7 scattering length*4 grid units)/7 grid units). In suchembodiments, the mapping of scattering length values to vertices isfacilitated as the scattering lengths of these grids are uniform withinthis region.

Next, the diffuse illumination values are determined by solving theelliptic radiative transfer equation on the grid values:

μ∇² D=I−αD

In this equation, μ is a local scattering coefficient, α is the localextinction coefficient, I is the source illumination from the surface,and D is the diffuse radiosity. In the present embodiment, μ and α (anextinction value) are derived from the attenuation factor grid valuesand the scattering length grid values; and I is derived from the diffuseillumination grid values. In this equation, αD represents the amount ofillumination I absorbed, and μ∇²D represents the illumination flux.

FIGS. 10A-D illustrate examples according to embodiments of the presentinvention. With embodiments of the present invention, to increase theamount of sub-surface scattering illumination (diffuse illuminationcompensation value) of a particular color, the scattering length of thatcolor is increased compared to the other colors. For example, when anear of a character is backlit, the ear should appear reddish. In thisexample, to implement this, the scattering length of red is increasedrelative to blue and green. FIG. 10A illustrates an initial amount ofscattering with respect to distance, for each color component in thisexample, assuming equal input intensity for each color. In FIG. 10A, thescattering lengths of red 1300, blue 1310, and green 1320 are initiallyapproximately the same. Next, in FIG. 10B, the scattering length of red1305 is increased relative to blue 1310 and green 1320.

As a side effect of increasing the scattering length of red 1305, theinventors discovered that in region 1330, the amount of scattering ofblue 1310 and green 1320 exceed the intensity of red 1305. Because ofthis, within region 1330, blue 1310 and green 1320 become thepredominant color over red 1305. However, in embodiments of the presentinvention, it is typically desired that the color of light having thelongest scattering length should also remain the predominant colorthrough out the range of the object.

In various embodiments of the present invention, to address this issue,the extinction coefficients of the undesired colors are increasedrelative to the desired predominant color. As is known, extinction iscommonly a sum of absorption and scattering. Because increasing theamount of scattering (decreasing scattering lengths) of blue 1310 andgreen 1320 would not change the fact that there would be a region whereblue and green have higher scattering than red, the inventors adjust theabsorption of blue 1310 and green 1320 of the material. As a result, asshown in FIG. 10C, by increasing the extinction coefficients (increasingthe absorption) of the material in blue and green, the scatteredintensity of blue 1340 and green 1350 remains lower than the intensityof red 1305 within region 1330. Accordingly, for a wide range ofthicknesses, the subsurface scattering illumination effect would bepredominately red.

In various embodiments of the present invention, a hybrid approach isused to address this issue. More specifically, in region 1370 in FIG.10D (corresponding to region 1330), the scattering lengths or extinctioncoefficients for the material in red, blue, and green are set to defaultproperty values. This results in the same scattered intensity plot asillustrated in region 1330 in FIG. 10A: within region 1330, the color isneutral, and not blue-green, as illustrated in region 1330 in FIG. 10B.Further, in region 1380 in FIG. 10D (after region 1370), the scatteringlengths or extinction coefficients for the material in red are increasedrelative to blue and green. This results in the scattered intensity plotas illustrated after region 1330 in FIG. 10B: in region 1380, thescattered illumination color should be reddish. In various embodiments,to reduce artifacts, a determination of diffuse illumination values asillustrated in FIG. 10A is performed separately from a determination ofdiffuse illumination values as illustrated in FIG. 10B. Subsequently,the results are combined, as illustrated in FIG. 10D. As illustrated inFIG. 10D, at point 1385, a dip in scattered intensity at the boundary isreplaced with a localized maximum 1387 for sake of continuity.

In still other embodiments of the present invention, the approachillustrated by the scattered intensity plot illustrated in FIG. 10B maybe used in conjunction with user-specifiable weights. In someembodiments, as illustrated in FIG. 11, a user adjustment control may beprovided to adjust the percentage or proportion of irradiance 1400 thatis attributed to the diffuse reflection 1410 and is used for thesub-surface scattering computations 1420. In some embodiments as alsoillustrated in FIG. 11, a user adjustment control may be provided toadjust the percentage or proportion contribution of a diffuse reflection1430 and diffuse illumination compensation value 1440 (exitant radiance)to the total illumination 1450. In various embodiments, for either orboth of the user adjustments, the “X” and “Y” percentages may beindependent from each other and can range from 0% to 100%. In otherembodiments, “X” and “Y” may be linked, for example, X may equal Y,X=2*Y, or the like. In still other embodiments, the ranges may belimited, e.g. 25% to 75%, or the like. In various embodiments, the usermay change these percentages by keyboard entry, knobs, sliders, or thelike. In various embodiments of the present invention, the user setadjustments (e.g. percentages) can be associated with the outermostsurface grids locations.

In various embodiments, by controlling the percentages and proportionsof the irradiance and/or controlling the percentages and proportions oftotal illumination, the blue-green artifact illustrated in FIG. 10B, forexample, can be reduced. For instance in regions where the diffuseillumination compensation value 1430 are blue-green in color, a greaterproportion of diffuse reflection 1420 may be used for total illumination1440. As a result of these scenarios, the blue-green sub-surfacescattering contribution is attenuated. Further, in regions where diffuseillumination compensation value 1430 are reddish in color, a greaterproportion of diffuse illumination compensation value 1430 may be usedfor total illumination 1440. As a result of this, the reddishsub-surface scattering contribution becomes more predominant. Anotherbenefit to such embodiments includes that multiple, separatedeterminations, discussed in FIG. 10D, are reduced.

In the present embodiment, the diffuse illumination values aredetermined by running a recursive low pass filter on the illuminationgrid values, step 390. In one embodiment, the recursive filter may besimilar to the following equation:

Y(n)=I(n)+α(n)Y(n−1)

In this equation, Y(n) represents a diffuse illumination compensationvalue at point n; I(n) represents the diffuse illumination value atpoint n; α(n) represents an extinction value at point n; and Y(n−1)represents the diffuse illumination compensation value at point n−1. Inthis embodiment, α(n) (derived from the scattering length and absorptionvalues) varies across the grid.

In this embodiment, the recursive low pass filter is run upon theillumination grid values in six directions, for example, +x, −x, +y, −y,+z, −z directions. By doing this, the illumination values areeffectively spread within the material to approximate the Green'sfunction of the scattering operator (μ∇²), while also compensating formaterial having different scattering lengths. In this embodiment, thevalues for α are selected to simulate Green's function. Moreparticularly, both the recursive low pass filter and Green's functionsare both exponential, thus α is selected such that the low pass filterhas approximately the same decay characteristics of Green's function. Inthe present embodiment, the change is a for adjacent samples is assumedto be small compared to the value of α.

In other embodiments, other numerical methods for determining thediffuse illumination grid are contemplated. For example, a fast ellipticsolver such as a fast convolution FFT solver or a multi-grid solver canbe applied using the illumination grid values and scattering gridvalues, described above. In light of the present disclosure, one ofordinary skill in the art will recognize that other methods for solvingfor the diffuse illumination values can be used.

In the present embodiment, a result of this process is a diffuseillumination grid where the vertices are populated with diffuseillumination compensation values (scattering illumination values)resulting from the spreading of light within an object. Moreparticularly, the diffuse illumination compensation values of surfacevoxels represent an amount of illumination to be added to the surfacediffuse illumination values determined by the surface shader. Forexample, if an translucent object is thin and is back-lit, the surfaceshader may specify a low diffuse illumination value at surface. However,using the process described above, the total diffuse illumination valueat the surface may be increased on account of the diffuse illuminationcompensation values determined above.

In the present example, the surface voxels determined above are againidentified, and the diffuse illumination compensation values forvertices in the surface voxels are interpolated to determine diffuseillumination compensation factors for each surface sample, step 400. Anyconventional interpolation process may be implemented. Next, the diffuseillumination compensation values are combined with the surface shadingvalues (diffuse illumination values) determined by the shader, step 410.In various embodiments of the present invention, the combination may beany linear or non-linear combination of the two values. In the presentembodiment, the resulting surface illumination values represent theappearance of the object illuminated by lighting sources compensated forthe object material.

In various embodiments of the present invention, the inventors of thepresent invention have discovered that performing subsurface scatteringof light in the method described in the above-referenced application mayunintentionally soften fine geometric details on the surface of theobject such as wrinkles, creases, bumps, and the like. To reduce sucheffects, weighted combinations of traditional surface shading values andsubsurface scattering values are used to determine the final value forthe surface.

In various embodiments, the respective combination of the diffuseillumination compensation factor (scattered light) and the diffuseillumination value (reflected light) for each surface sample, is basedupon a weighting factor. In some embodiments, the weighting factor maybe the same for all surface samples. As an example, the combined diffuseillumination values for all surface points are based upon 60% of thediffuse illumination values and 40% of the diffuse illuminationcompensation values, 75% and 25%, or any other proportion. In otherembodiments, the weighting factors may be the same or different for thesurface samples. For example, the combined diffuse illumination valuesfor a first and a second surface point may be based upon a ratio of 3:1of the diffuse illumination value to diffuse illumination compensationvalue, whereas a third surface point may be based upon a ratio of 1:5.In other examples, other proportions are contemplated. As discussedabove, in various embodiments, the weighting or proportions may be userselectable by keyboard, slider, knob, or the like.

In various embodiments, the weighting factors may be automatically(procedurally) determined, determined based upon input from a user, or acombination of these methods. In various implementations, the inventorsof the present invention have discovered that in areas with finegeometric details, such as a corner of a mouth, crease of an eye, or thelike, the diffuse illumination value should have a higher weight.Further, in areas with relatively little geometric fluctuations, such asa cheek or forehead, the sub-surface scattering (diffuse illuminationcompensation values) values may have a higher weight. In one embodiment,the system may automatically determine areas of the object with high andlow geometric complexity. Then in areas of high geometric complexity,the system assigns a smaller weight to the surface points compared tosurface points within areas of low geometric complexity. As a result, inareas of high geometric complexity, the percentage or ratio of thediffuse illumination compensation values compared to the diffuseillumination values are small compared to surface points within areas oflow geometric complexity.

In one embodiment, manual methods may also be used by a user to setweights for surface points. FIG. 6 illustrates a process according to anembodiment of the present invention. More specifically, FIG. 6illustrates a paint-based method to set weights. Initially, atwo-dimensional view of the three dimensional object is formed, step700. In various embodiments, the views may be user-defined views, orpredetermined views, such as a top view, front view, and the like. Next,using a two-dimensional paint-type program, a user “paints” atwo-dimensional image in an overlay layer over the two-dimensional view,step 710. Any conventional paint-type program may be used, such as AdobePhotoshop, Jasc PaintShop Pro, or the like.

Next, in various embodiments, the two-dimensional image is projectedback into three-dimensional space, step 720. Specifically, the pixelvalues in the two-dimensional image are projected back to surfacepositions of the object. Feathering techniques may be used to smooth theprojection. Additionally, more than one two-dimensional view may be usedfor the paint-projection process. Further details regardingtwo-dimensional paint-based methods are disclosed in PCT/US04/08993filed Mar. 23, 2004, incorporated by reference for all purposes.

In various embodiments, the pixel values in the two-dimensional imageare then used to represent weighting factors for the surface positions,as discussed above in step 410. For example, in this method, the pixelvalues relate to the amount of the diffuse reflection and the amount ofthe sub-surface scattering values for the surface position. For instancea pixel value of 255 (selected from {0 . . . 255}) indicates that thesurface position includes only the diffuse reflection; a pixel value of0 indicates that the surface position includes only the sub-surfacescattering values; a pixel value of 128 indicates that the surfaceposition includes approximately 50% of the diffuse reflection and 50% ofthe diffuse illumination compensation value, and the like.

FIGS. 7A-B illustrate examples according to embodiments of the presentinvention. In FIG. 7A, a three-dimensional object 800 includes areas 810and 820 of high geometric complexity, and a thinner area 830. In theembodiment in FIG. 7B, a two-dimensional view 840 of object 800 isopened within a paint-type program. In this example, a user paints in anoverlay layer, geometric object 850 and 860. In the present embodiment,the value of the paint is associated with a low proportion of subsurfacescattering values compared to the diffuse illumination. Additionally, inthis example, the user paints geometric object 870. In the presentembodiment, the value of the paint is associated with a high proportionof subsurface scattering values compared to the diffuse illumination.For portions of object 800 not associated with painted weight values, adefault proportion of subsurface scattering values and diffuseillumination values, such as 50% each, may be set by the user.

According to the embodiment above, the paint values in the overlay layerare then mapped back to the three-dimensional model. Accordingly, paintvalues of geometric object 850 and 860 are then mapped back to areas 810and 820, respectively; and paint values of geometric object 870 aremapped back to thinner area 830.

In other embodiments of the present invention, the inventors havediscovered that in some regions of an object, the sub-surface scatteringvalues may unintentionally “leak” light because of the geometry of theobject. In such cases, instead of modifying and re-running thesub-surface scattering process, or providing an updated description ofthe geometric object and running the sub-surface scattering process, itis simpler to “ignore” the light leaks, by weighting the diffusereflection values much higher than the sub-surface scattering values(e.g. 90% versus 10%), or the like in those areas.

In additional embodiments of the present invention, the inventors haverecognized that the use of wavelength dependant scattering typicallyalters the hue of surface colors. However, this sometimes conflicts withthe surface color and surface texture of an object that arepainstakingly determined by one or more object modelers, artisticdirectors, or the like. Accordingly, the inventors have recognized thatthe use of sub-surface scattering illumination on surfaces, withoutregard to the surface color may conflict with the determined surfacecolor. Although, using sub-surface scattering illumination as describedabove may result in a more technically accurate result, the inventorshave determined that attempting to maintain the surface color is moreimportant in various embodiments.

FIGS. 8A-B illustrate examples according to embodiments of the presentinvention.

FIG. 8A illustrates an example where tinting of light occurs prior toscattering. More specifically, FIG. 8A illustrates a two-dimensionalcross-section of an object 900 having a first surface 910 of aorange-colored material (e.g. r,g,b=230, 121, 121 or r,g,b=(0.9, 0.47,0.47), and a second surface 920 that has a yellow-color material (e.g.r,g,b=144, 144,9 or r,g,b=0.56, 0.56, 0.04). Also shown is anwhite-light (e.g. r,g,b=196, 196, 196 or r,g,b=0.77, 0.77, 0.77)illumination source 930 primarily illuminating first surface 910. Inthis example, the diffuse illumination value at a surface point 940 isorange-ish (e.g. 0.77, 0.36, 0.36), and the diffuse illumination valueat surface point 950 is dark yellow (e.g. 0.09, 0.09, 0.01).

According to one embodiment of the present invention, the diffuseillumination value (0.77, 0.36, 0.36) is used to perform the sub-surfacescattering operation to determine diffuse illumination compensationvalues. In this example, at surface point 950, the diffuse illuminationcompensation value is (0.39, 0.01, 0.01). Then, assuming a weightedcombination of 75% diffuse illumination value and 25% diffuseillumination compensation value at surface point 950, the compensateddiffuse illumination value is approximately (0.17, 0.09, 0.01). As canbe determined, this value is more orange than yellow in color, althoughsecond surface should be more yellowish in color.

In various embodiments of the present invention, in step 320 in FIG. 2A,in addition to determining the diffuse illumination values at surfacepositions on the object, the surface shading calculations includedetermining incident illumination at surface positions on the object.Additionally, the material colors at the surface positions may be knownor determined through any number of shading techniques such asprocedural textures or painted bitmap mappings.

Then, in various embodiments, prior to steps 350-400, the incidentillumination at the surface positions are scaled by the respectivematerial colors of the surface positions to determine a color for thescattering illumination calculations. In one specific embodiment, theincident illumination is multiplied by the square root of the inputsurface color and then scaled. As an example, using color values from 0to 255, if the incident light is gray light (e.g. (r,g,b)=(196, 196,196)), and the input surface color is bluish (e.g. 144, 144, 225), theincident light (196, 196, 196) is multiplied by a square root of theinput surface color (e.g. 12, 12, 15) and then scaled. For example, ifthe scaling factor is 15, the resulting color for the scatteringillumination calculation, the scattering illumination color isapproximately (156,156, 196), which is approximately blue in color. Asdiscussed above, in various embodiments, steps 350-400 may then beindividually performed for the separate component colors (e.g. r,g,b)because the object material may absorb or scatter the component colorsat different rates.

In various embodiments of the present invention, after determining theillumination values at surface positions due to the subsurfacescattering, the illumination values are scaled by the respectivematerial colors of the output surface positions to determine a color forthe diffuse illumination compensation values. Similar to the above, inone specific embodiment, the scattering illumination is multiplied bythe square root of the output surface color and then scaled. As anexample, if the value of the subsurface scattering at a surface point isyellowish in color (e.g. 100, 100, 10), and the material surface isgreenish (9, 244, 100). In this example, the subsurface scattering valueof (100 100,10) is multiplied by a greenish value (e.g. 3,12,10) andthen scaled. For example, if the scaling factor is 12, the resultingcolor for the illumination compensation value is approximately(25,100,83), which is greenish in color, matching the color of thematerial surface.

The above is merely an example of a process according to embodiments ofthe present invention. In one implementation the color values are scaledfrom 0 to 1. In such implementations, the resulting color values aredifferent than when the color values are scaled from 0 to 255, or thelike.

FIG. 8B illustrates a two-dimensional cross-section of an object 1000similar to FIG. 8A. FIG. 8B illustrates an example where tinting oflight occurs prior to scattering and after scattering (e.g. via squareroot multiplication). In this example, a first surface 1010 of aorange-colored material (e.g. r,g,b=0.9, 0.47, 0.47), and a secondsurface 1020 has a yellow-color material (e.g. r,g,b=0.56, 0.56, 0.04).Also shown is an white-light (e.g. r,g,b=0.77, 0.77, 0.77) illuminationsource 1030 primarily illuminating first surface 1010. In this example,the diffuse illumination value at a surface point 1040 is orange-ish(e.g. 0.73, 0.50, 0.50), and the diffuse illumination value at surfacepoint 1050 is also dark yellow (e.g. 0.56, 0.56, 0.01).

According to one embodiment of the present invention, illuminationsource 1030 (0.77, 0.77, 0.77) is a multiplied with the square root ofthe orange-colored material: (0.95, 0.69, 0.69) to obtain theillumination value (0.73, 0.50, 0.50). After the sub-surface scatteringoperation, the subsurface scattering value at point 1050 is (0.37, 0.13,0.01). According to the above embodiment, this value is then multipliedby the square root of the yellow-colored material: (0.56, 0.56, 0.56),to obtain the diffuse illumination compensating value is (0.28, 0.10,0.01).

Next, assuming a weighted combination of 75% diffuse illumination valueand 25% diffuse illumination compensation value at surface point 1050,the compensated diffuse illumination value is approximately (0.13, 0.10,0.01). In the present example, the value of (0.13, 0.10, 0.01) is moreyellow than the value of (0.17, 0.09, 0.01) determined for surface point950, in FIG. 8A.

FIGS. 9A-C illustrate additional examples according to embodiments ofthe present invention. FIG. 9A illustrates an example where tinting oflight occurs prior to scattering. More specifically, FIG. 9A illustratesa two-dimensional cross-section of an object 1100 having a surface 1110of a given color 1115 (e.g. r,g,b=0.8, 0.7, 0.6 from a range for each of{0 to 1}). Also shown is an white-light (e.g. 1.0, 1.0, 1.0)illumination source 1120 primarily illuminating first surface 1110. Inthis example, the initial diffuse illumination 1140 for the sub-surfacescattering effect (e.g. 0.8, 0.7, 0.6) and a diffuse reflection 1145 isshown (e.g. 0.8, 0.7, 0.6). After sub-surface scattering computations,the diffuse illumination compensation value (scattering illumination)1150 at surface point 1160 is determined, (e.g. 0.8, 0.5, 0.4). Next,assuming a weighted combination of 75% the initial diffuse illuminationcompensation value 1150 and 25% of a diffuse reflection (in thisexample, the same as diffuse reflection 1145) at surface point 1150, thecompensated diffuse illumination value (combination of diffusereflection and sub-surface scattering value) 1170 is determined (e.g.0.8, 0.65, 0.55). As can be determined, this value 1170 is more red thanthe original given color 1115 (e.g. 0.8, 0.7, 0.6).

FIG. 9B illustrates an example where tinting of light occurs prior toscattering and after scattering (e.g. via square root multiplication).FIG. 9B illustrates a two-dimensional cross-section of an object 1200having a surface 1210 of a given color 1215 (e.g. r,g,b=0.8, 0.7, 0.6from a range for each of {0 to 1}). Also shown is an white-light (e.g.1.0, 1.0, 1.0) illumination source 1220 primarily illuminating firstsurface 1210. In this example, the diffuse reflection 1245 is shown(e.g. 0.8, 0.7, 0.6), and the initial diffuse illumination 1240 for thesub-surface scattering effect is shown (e.g. 0.9, 0.8, 0.7) aftermultiplied by the square root of the given color 1215. After sub-surfacescattering computations, scattering illumination contribution 1250 isdetermined, (e.g. 0.85, 0.65, 0.55). As can be seen, the scatteringillumination contribution is multiplied by the square root of the givencolor 1215, to obtain the diffuse illumination compensation value 1265at point 1260. Next, assuming a weighted combination of 75% the diffusereflection 1245 (in this example, same as diffuse reflection 1145: (0.8,0.7, 0.6) and 25% of the diffuse illumination compensation value 1265 atsurface point 1260, the compensated diffuse illumination value(combination of diffuse reflection and sub-surface scattering value)1270 is determined (e.g. 0.79, 0.66, 0.56). As can be determined, thisvalue 1270 is more similar to the original given color 1215 (e.g. 0.8,0.7, 0.6), compared to compensated diffuse illumination value 1270 (e.g.0.8, 0.65, 0.55).

FIG. 9C illustrates an example where tinting of light occurs afterscattering. In such embodiments, the inventors have noted that higherfrequency features of an object surface, e.g. freckles, wrinkles, arevisually preserved. FIG. 9C illustrates a two-dimensional cross-sectionof an object 1500 having a surface 1510 of a first color 1515 (0.8, 0.7,0.6) (e.g. a skin color) and a second color 1517 (0.6, 0.5, 0.25) (e.g.a freckle color). Also shown is an white-light (e.g. 1.0, 1.0, 1.0)illumination source 1520 primarily illuminating surface 1510. In thisexample, the diffuse reflection 1545 is shown (e.g. 0.8, 0.7, 0.6) andthe diffuse reflection 1547 is shown (e.g. 0.6, 0.5, 0.25). In thisexample, the initial diffuse illumination 1540 for the sub-surfacescattering effect is shown (e.g. 1.0, 1.0, 1.0). After sub-surfacescattering computations, scattering illumination contribution 1550 isdetermined, (e.g. 0.97, 0.76, 0.73). As can be seen, scatteringillumination contribution 1550 is multiplied by first color 1515 toobtain the diffuse illumination compensation value 1565 (0.78, 0.53,0.44), and scattering illumination contribution 1550 is multiplied bysecond color 1517 to obtain the diffuse illumination compensation value1567 (0.58, 0.38, 0.18). As can be seen, by filtering upon second color1517 (0.6, 0.5, 0.25), the color of diffuse illumination compensationvalue 1567 (0.58, 0.38, 0.18) can be roughly preserved.

As discussed above, in various embodiments, a user may modify thepercentage contribution of diffuse reflection and scatteringillumination contribution at specific points on a surface. However, forsake of continuing the examples of FIGS. 9A-b, it is assumed that totalillumination is derived from a weighted combination of 75% of thediffuse reflection, and 25% scattering illumination contribution. Morespecifically, the total illumination 1570 is (0.8, 0.66, 0.56), and thetotal illumination 1572 is (0.6, 0.47, 0.23). As can be seen in thisexample, the coloring of second color 1517 (0.6, 0.5, 0.25) is roughlypreserved in total illumination 1572. As a result of this, the surfaceappearance of the object (e.g. mole, freckle) is roughly preserved.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. Many changes ormodifications are readily envisioned. For example, some embodiments ofthe present invention allow the user to simulate scattering of light ina material in a non-uniform manner. This is also known as anisotropicscattering. Typically, the irradiance gathering stage discards thedirectionality of incident light intensities, accordingly, scatteringdirectionality is impossible. However, it is possible to achieve someamount of scattering directionality by recording the irradiance samplesat surface locations and moving the values along the vector defined bythe incident light sources to the surface locations.

In the various embodiments, to simulate this process, the diffuseillumination values are modified within the illumination grid prior tothe scattering phase, described above. More specifically, the diffuseillumination values determined at voxels bounding surface points may bemoved forward into the object or moved backward outside the objectbetween steps 360 and 365.

FIGS. 5A and 5B illustrate examples according to embodiments of thepresent invention. In the example in FIG. 5A, diffuse illuminationvalues 605 due to lighting source 600 for each vertex in the voxel 640are shown. Additionally, diffuse illumination values 615 due to lightingsource 610 for each vertex in voxel 640 are also shown. An illuminationgrid 650 representing voxels discussed above is also shown.

In FIG. 5B, forward scattering is illustrated with voxel 630. As can beseen, the diffuse illumination values 605 due to lighting source 600 aremoved from voxel 605 to voxel 630. In the various embodiments, thedistance of voxel 630 from 605 in the direction away from illuminationsource 600 is typically estimated by a user, based upon the propertiesof the object material. For example, if the object material is moretranslucent in the direction of the light, the offset is deeper into theobject; and if the object material is less translucent in the directionof the light, the offset is shallower into the object.

In FIG. 5B, reverse scattering is illustrated with voxel 620. As can beseen, the diffuse illumination values 615 due to lighting source 610 aremoved from voxel 605 to voxel 620. In the various embodiments, thedistance of voxel 630 from 620 in the direction towards illuminationsource 600 is typically estimated by a user, based upon the propertiesof the object material. For example, if the object material is moreopaque in the direction of the light, the offset is further away fromthe object; and if the object material is less opaque in the directionof the light, the offset is closer to the object.

In various embodiments of the present invention, after compensating forforward scattering for all light sources, as shown, for example in FIG.5B, steps 365-410 may be performed, as previously discussed.

In various embodiments of the present invention, the inventors havedetermined that advantages of the present schema may be applied tovarious rendering pipelines. For example, some embodiments may becombined with the existing RenderMan® Reyes rendering architecture.Other embodiments may also be combined with the shot renderingtechniques described in the above-referenced application. Further, it isenvisioned that a great number of shaders currently compatible withRenderMan® could be used in various embodiments of the presentinvention.

In some embodiments of the present invention, the subsurface scatteringeffects discussed above, may or may not be desired for all renderings ofa scene. For example, in some embodiments, the generation of theseeffects may be more time consuming than desired by a user, i.e. they maynot be real-time processes. Accordingly, in some embodiments, one ormore switches may be included in the rendering pipeline that bypassesthe techniques described above.

In various embodiments of the present invention, switching on and off ofthe subsurface scattering effects described above may be made ondifferent levels of granularity. For example, scattering effects may beswitched on or off for different parts of object models, differentobject models, different scenes, different shots, and the like.Additionally, the switches may allow the user to globally determine apercentage of scattered illumination contributions and diffuseillumination contributions.

In some embodiments, a simpler simulation of the subsurface scatteringprocess may be used to provide an approximation of the subsurfacescattering, when the techniques described above are turned off. Forexample, a diffuse reflection in the illumination model may be used. Asanother example, the techniques described in U.S. patent applicationSer. No. 10/810,064 filed Mar. 25, 2004 may be used.

Embodiments of the present invention may be applied to any number ofrendering platforms and for a variety of purposes. For example,embodiments may be use on engineering workstations for developmentpurposes, on visualization systems for artists and animators, in arendering farm machine for final production, and the like. Accordingly,the concepts disclosed above are extremely valuable in a variety ofapplications.

In the above disclosure, any resolution for the grids may be used. Theinventor has determined that it is advantageous that the grid be largerthan the smallest shaded surface. Further, the grid may be much greaterin size. In one embodiment, a typical voxel may include two shadingsurface values to nine shading surface values. In other embodiments,voxels may include a smaller number of shading surface values, e.g. 0.5or greater, and voxels may include a larger number of shading surfacevalues, e.g. 10 to 25, 20 to 40, 30 or greater, and the like.

In various embodiments of the present invention, because the resolutionfor the grids may vary based upon the number of shaded surfaces of theobject, the resolution may also be said to vary based upon placement ofthe object within a scene. This is because the inventors of the presentinvention recognize that the number of shading surfaces of an objecttypically can vary according to placement of the object within a scene.In other words, if the object is close to the viewer, the number ofshading surfaces of the object increases, otherwise the object wouldlook blocky, the surface appearance would include unwanted artifacts,and the like. Similarly, as the object is farther away from the viewer,the number of shading surfaces decreases to reduce the number ofunnecessary rendering computations. Thus, as the number of shadingsurfaces increase or decrease for the object, the number or resolutionof the volumetric grids, or voxels can also vary. As merely an example,a volumetric grid for subsurface scattering for a spherical object mayhave a bounding box including 1000×1000×1000 voxels when the object isin the foreground of a scene, and the same object may have a boundingbox including 100×100×100 voxels when the object is in the background ofa scene.

Additional Control Features

According to various embodiments, new control features are provided toenhance a user's ability to soften or sharpen surface features and tocontrol the amount of subsurface scattering that contributes to diffuseillumination. Such control features provide more photorealistic effectsand are particularly useful for rendering objects having translucentqualities, such as fleshy humanoids.

To capture subsurface scatter in plastics or skin or other materialshaving translucent properties, a pre-pass render is calculated tocapture direct illumination values from surrounding light sources. Thedirect illumination is then saved into a uniform voxel structure whichallows for easy filtering for the three color channels. By blurring thedirect illumination in the voxel data structure, this approximates theeffect of light scattering within the surface. In the final render, thefiltering values are read in from the data structure and added to thedirect diffuse component of the illumination. It is desirable, however,to provide a control to vary how much light is filtered entering thesurface vs. exiting the surface. In one embodiment, therefore, a user isable to control the softness, or sharpness, of a surface feature using asubsurface softening control parameter. In one embodiment, thesubsurface softening control parameter allows a user to specify theamount of filtering or blurring. By tinting the direct illuminationbefore blurring, this would soften surface features, and by tinting theresult of the blurring, this would sharpen the surface features. Ahigher value would favor tinting before blurring: a lower value; afterblurring. For example, returning to FIG. 8A, for example, if a user todesires to sharpen a surface feature at point 950, then user wouldselect a lower parameter value that favors tinting point 950 afterblurring. Similarly, if a user desires to soften a feature at point 950,the user would select a parameter value that favors tinting point 940before blurring. In certain aspects, the subsurface softness controlparameter includes a sliding value (e.g., between 0% and 100%) thatallows for favoring filtering of the incoming light (e.g., at point 940)and the outgoing light (e.g., at point 950) while maintaining theaccumulated amount of filtering; e.g., to provide sharper detail atpoint 950 a user might select a parameter value that filters 20% atpoint 940 and 80% at point 950. In essence, the subsurface softnesscontrol parameter allows a user to favor whether incoming light istinted or outgoing light is tinted to produce sharpened or softenedfeatures while preserving the filter response.

According to another embodiment, rather than having the diffusecomponent of illumination for a surface feature having a set of one ormore pixels be determined as either all subsurface scattering or alldirect reflection, a user is provided with an ability to control theamount of specular reflection and/or direct reflection included in thediffuse light component. For example, for a shading system whereillumination of a surface point or surface feature is made up of adiffuse lighting component, and other components such as a reflectioncomponent and a specular reflection component, it is desirable to tunethe amount of subsurface scattering light that contributes to thediffuse component. Thus, in one embodiment, a user such as a shadingartist or other user may set the diffuse lighting component at a certainpercentage (e.g., 0% to 100%) and a user is able to control the amountof subsurface scatter light that makes up the diffuse component. Forexample, where a user has set the diffuse light component to make up 60%of the illumination for a surface point or set of surface points, a usercan control the amount of subsurface scattering illumination that makesup that amount (e.g., 0% to 100% subsurface scatter), with the remaindermade up of direct reflection of illumination at the surface point(s).The subsurface scatter and direct reflection components are added toproduce the diffuse light component (e.g., 60% of illumination), whichare then added to the remaining components (40%) such as specular andreflection components. In this embodiment, the subsurface scatteringcomponent and direct reflection components of diffuse light can be tunedto provide more photorealistic effects.

According to another embodiment, a method of accumulating weightedaverages for different illumination controls is provided. Such methodsare particularly useful where a material is composed of differentlayers, each with different subsurface scattering properties, e.g.,different subsurface scattering responses. An example of an objecthaving multiple layers would be a skin material, with layers for theepidermis and the dermis. In this embodiment, rather than creating aseparate scatter file for each layer of the surface material, a singlescatter file is created for the surface to be rendered. For example, itis desirable to allow different bleeding or blur properties fordifferent layers; this would be more difficult and expensive to achievewith separate scatter files. In one embodiment, the material propertiesof the different layers are composited using a subsurface scatteringthickness ratio for the layers. For example, light may spend more timein thicker (dermis) material than in thinner (epidermis) material;therefore more weight should be given to the thicker dermis layer. Thesubsurface scattering properties are then blended together based on theweight ratios, e.g., added together with a weighted average. Forexample, for a bleed parameter, the bleed parameters of each layer arecombined based on the thickness ratios of the layers. Similarly, othercontrol parameters such as scatter tint and subsurface scattering, asdiscussed above, are blended based on the weighted averages of thelayers. Thus, in certain aspects, all illumination controls can beaccumulated together based on a weighted average (thickness ratios).

In other embodiments of the present invention, the volumetricillumination processing (e.g. subsurface scattering) may be applied toother parameters. For example, the volumetric processing may be used todetermine surface textures, may be used as parameters for filtering(e.g. low-pass filtering in areas of high light scattering), may be usedas inputs for shading (e.g. boosting or decreasing saturation, boostingor decreasing lightness, changing of hue based upon amount of subsurfacescattering at certain regions). In light of the above disclosure, theinventors believe that many other types of uses for the volumetricprocessing are now obvious to one of ordinary skill in the art.

Further embodiments can be envisioned to one of ordinary skill in theart after reading the attached documents. In other embodiments,combinations or sub-combinations of the above disclosed embodiments canbe advantageously made. The block diagrams of the architecture and flowcharts are grouped for ease of understanding. However it should beunderstood that combinations of blocks, additions of new blocks,re-arrangement of blocks, and the like are contemplated in alternativeembodiments of the present invention.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that various modifications and changes may be made thereuntowithout departing from the broader spirit and scope of the invention asset forth in the claims.

Subject matter of claims may include variable weighting of diffusereflection and scattered radiance in different combinations along thesurface of an object:

1. A method for a computer system comprising:

determining a first diffuse illumination compensation value associatedwith a first surface region on an object in response to an irradianceassociated with the first surface region;

determining a first diffuse reflection value associated with the firstsurface region on the object in response to the irradiance associatedwith the first surface region;

determining a second diffuse illumination compensation value associatedwith a second surface region on the object in response to an irradianceassociated with the second surface region;

determining a second diffuse reflection value associated with the secondsurface region on the object in response to the irradiance associatedwith the second surface region;

receiving a specification of a first weighting for the first diffuseillumination compensation value and the first diffuse reflection value,and a second weighting for the second diffuse illumination compensationvalue and the second diffuse reflection value, wherein the firstweighting is different from the second weighting;

determining a diffuse illumination value associated with the firstsurface region on the object in response to the first diffuseillumination compensation value, the first diffuse reflection, and thefirst weighting;

determining a diffuse illumination value associated with the secondsurface region on the object in response to the second diffuseillumination compensation value, the second diffuse reflection, and thesecond weighting;

determining an image including a rendered representation of the objectin response to the diffuse illumination value associated with the firstsurface region and to the diffuse illumination value associated with thesecond surface region;

displaying the image to the user; and

storing a representation of the image on a tangible media.

2. A computer system comprising:

a memory configured to store a model of an object having a first surfaceregion and a second surface region; and

a processor coupled to the memory, wherein the processor is configuredto determine a first diffuse illumination compensation value associatedwith the first surface region in response to an irradiance associatedwith the first surface region, wherein the processor is configured todetermine a first diffuse reflection value associated with the firstsurface region in response to the irradiance associated with the firstsurface region, wherein the processor is configured to determine asecond diffuse illumination compensation value associated with a secondsurface region in response to an irradiance associated with the secondsurface region, wherein the processor is configured to determine asecond diffuse reflection value associated with the second surfaceregion in response to the irradiance associated with the second surfaceregion, wherein the processor is configured to receive a specificationof a first weighting for the first diffuse illumination compensationvalue and the first diffuse reflection value, and a second weighting forthe second diffuse illumination compensation value and the seconddiffuse reflection value, wherein the first weighting is different fromthe second weighting, wherein the processor is configured to determine adiffuse illumination value associated with the first surface region inresponse to the first diffuse illumination compensation value, the firstdiffuse reflection, and the first weighting, wherein the processor isconfigured to determine a diffuse illumination value associated with thesecond surface region in response to the second diffuse illuminationcompensation value, the second diffuse reflection, and the secondweighting, and wherein the processor is configured to determine an imageincluding a rendered representation of the object in response to thediffuse illumination value associated with the first surface region andto the diffuse illumination value associated with the first surfaceregion; and

wherein the memory is also configured to store the image.

Subject matter of claims may include scattering properties of differentlayers of materials being averaged:

1. A computer system comprising:

a memory configured to sore a specification of a three-dimensionalobject including a surface region associated with a plurality ofmaterials including a first material and a second material, wherein thefirst material is associated with a first thickness and is associatedwith a first value for a subsurface scattering parameter wherein thesecond material is associated with a second thickness and is associatedwith a second value for the subsurface scattering parameter, wherein thefirst value is different from the second value; and

a processor configured to determine a third value for the subsurfacescattering parameter associated with the first material and the secondmaterial in response to the first thickness, the first value for thesubsurface scattering parameter, the second thickness, and the secondvalue for the subsurface scattering parameter, and wherein the processoris configured to determine a diffuse illumination compensation valueassociated with the surface region in response to the third value forthe subsurface scattering parameter.

1. A method for a computer system comprises:

receiving a description of a three-dimensional scene including aspecification of placement of an object within the three-dimensionalscene;

receiving a three-dimensional model of an object;

determining a resolution for a volumetric grid comprising a plurality ofvoxels associated with the three-dimensional model of the object inresponse to the placement of the object within the three-dimensionalscene, and in response to the three-dimensional model of the object,wherein each voxels from the plurality of voxels is associated with asubsurface scattering parameter;

determining an incident irradiance associated with a plurality of voxelsassociated with a surface region of the object;

determining a subsurface scattering contribution associated with theplurality of voxels associated with a surface region of the object inresponse to the incident irradiance and in response to the subsurfacescattering parameters associated with the voxels from the plurality ofvoxels;

displaying the image to the user; and

storing a representation of the image on a tangible media.

2. A computer system comprises:

a description of a three-dimensional scene including a specification ofplacement of an object within the three-dimensional scene; and

receive a three-dimensional model of an object, wherein the processor isconfigured to determine a resolution for a volumetric grid comprising aplurality of voxels associated with the three-dimensional model of theobject in response to the placement of the object within thethree-dimensional scene, and in response to the three-dimensional modelof the object, wherein each voxels from the plurality of voxels isassociated with a subsurface scattering parameter, wherein the processoris configured to determine an incident irradiance associated with aplurality of voxels associated with a surface region of the object, andwherein the processor is configured to determine a subsurface scatteringcontribution associated with the plurality of voxels associated with asurface region of the object in response to the incident irradiance andin response to the subsurface scattering parameters associated with thevoxels from the plurality of voxels.

1. A method for a computer system comprising: determining a firstdiffuse illumination compensation value associated with a first surfaceregion on an object in response to an irradiance associated with thefirst surface region; determining a first diffuse reflection valueassociated with the first surface region on the object in response tothe irradiance associated with the first surface region; determining asecond diffuse illumination compensation value associated with a secondsurface region on the object in response to an irradiance associatedwith the second surface region; determining a second diffuse reflectionvalue associated with the second surface region on the object inresponse to the irradiance associated with the second surface region;receiving a specification of a first weighting for the first diffuseillumination compensation value and the first diffuse reflection value,and a second weighting for the second diffuse illumination compensationvalue and the second diffuse reflection value, wherein the firstweighting is different from the second weighting; determining a diffuseillumination value associated with the first surface region on theobject in response to the first diffuse illumination compensation value,the first diffuse reflection, and the first weighting; determining adiffuse illumination value associated with the second surface region onthe object in response to the second diffuse illumination compensationvalue, the second diffuse reflection, and the second weighting; anddetermining an image including a rendered representation of the objectin response to the diffuse illumination value associated with the firstsurface region and to the diffuse illumination value associated with thesecond surface region.
 2. The method of claim 1 wherein determining thefirst diffuse illumination compensation value associated with the firstsurface region on the object is also in response to a plurality ofvoxels having associated subsurface scattering parameters; and whereindetermining the second diffuse illumination compensation valueassociated with the second surface region on the object is also inresponse to the plurality of voxels having associated subsurfacescattering parameters.
 3. The method of claim 2 wherein the associatedsubsurface scattering parameters are selected from a group consistingof: extinction coefficients, scattering lengths.
 4. The method of claim1 wherein receiving the specification of the first weighting comprisesreceiving the specification in response to a user input; wherein theuser input is selected form a group consisting: keyboard entry,graphical selection with a user input device.
 5. The method of claim 1wherein the first weighting for the first diffuse illuminationcompensation value and the first diffuse reflection value comprises agreater percentage of the first diffuse illumination compensation valuecompared to the first diffuse illumination compensation value; whereinthe second weighting for the second diffuse illumination compensationvalue and the second diffuse reflection value comprises a smallerpercentage of the first diffuse illumination compensation value comparedto the first diffuse illumination compensation value.
 6. The method ofclaim 1 further comprising: displaying the image to the user; andstoring a representation of the image on a tangible media.
 7. A computerprogram product for a computer system comprising a processor and amemory, wherein the computer program product comprises acomputer-readable tangible media comprising executable computer codeincluding: code that directs the processor to determine a first diffuseillumination compensation value associated with a first surface regionof an object in response to an irradiance associated with the firstsurface region; code that directs a processor to determine a firstdiffuse reflection value associated with the first surface region inresponse to the irradiance associated with the first surface region;code that directs a processor to determine a second diffuse illuminationcompensation value associated with a second surface region of the objectin response to an irradiance associated with the second surface region;code that directs a processor to determine a second diffuse reflectionvalue associated with the second surface region in response to theirradiance associated with the second surface region; code that directsa processor to receive a specification of a first weighting for thefirst diffuse illumination compensation value and the first diffusereflection value, and a second weighting for the second diffuseillumination compensation value and the second diffuse reflection value,wherein the first weighting is different from the second weighting; codethat directs a processor to determine a diffuse illumination valueassociated with the first surface region in response to the firstdiffuse illumination compensation value, the first diffuse reflection,and the first weighting; code that directs a processor to determine adiffuse illumination value associated with the second surface region inresponse to the second diffuse illumination compensation value, thesecond diffuse reflection, and the second weighting; and code thatdirects a processor to determine an image including a renderedrepresentation of the object in response to the diffuse illuminationvalue associated with the first surface region and to the diffuseillumination value associated with the first surface region.
 8. A methodfor a computer system comprising: receiving a specification of athree-dimensional object including a surface region associated with aplurality of materials including a first material and a second material,wherein the first material is associated with a first thickness and isassociated with a first value for a subsurface scattering parameterwherein the second material is associated with a second thickness and isassociated with a second value for the subsurface scattering parameter,wherein the first value is different from the second value; determininga third value for the subsurface scattering parameter associated withthe first material and the second material in response to the firstthickness, the first value for the subsurface scattering parameter, thesecond thickness, and the second value for the subsurface scatteringparameter; determining a diffuse illumination compensation valueassociated with the surface region in response to the third value forthe subsurface scattering parameter.
 9. The method of claim 8 whereindetermining the third value for the subsurface scattering parametercomprises determining a weighted thickness average of the first valuefor the subsurface scattering parameter and the second value for thesubsurface scattering parameter.
 10. The method of claim 9 whereindetermining the diffuse illumination compensation value comprisesperforming a volumetric subsurface scattering computation in response tothe third value for the subsurface scattering parameter, to the firstthickness, and to the second thickness.
 11. The method of claim 10wherein the first material and the second material underlies the surfaceregion of the three-dimensional object; wherein the first material isassociated with a first volumetric region; and wherein the secondmaterial is associated with a second volumetric region.
 12. The methodof claim 10 wherein performing the volumetric subsurface scatteringcomputation comprises performing a recursive filtering operation inresponse to the third value for the subsurface scattering parameter. 13.The method of claim 8 further comprising determining an image includinga rendered representation of the three-dimensional object in response tothe diffuse illumination value associated with the surface region.
 14. Acomputer program product for a computer system comprising a processorand a memory, wherein the computer program product comprises acomputer-readable tangible media comprising executable computer codeincluding: code configured to direct the processor to receive aspecification of a three-dimensional object including a surface regionassociated with a plurality of materials including a first material anda second material, wherein the first material is associated with a firstthickness and is associated with a first value for a subsurfacescattering parameter wherein the second material is associated with asecond thickness and is associated with a second value for thesubsurface scattering parameter, wherein the first value is differentfrom the second value; code configured to direct the processor todetermine a third value for the subsurface scattering parameterassociated with the first material and the second material in responseto the first thickness, the first value for the subsurface scatteringparameter, the second thickness, and the second value for the subsurfacescattering parameter; and code configured to direct the processor todetermine a diffuse illumination compensation value associated with thesurface region in response to the third value for the subsurfacescattering parameter.
 15. A method for a computer system comprises:receiving a description of a three-dimensional scene including aspecification of placement of an object within the three-dimensionalscene; receiving a three-dimensional model of an object; determining aresolution for a volumetric grid comprising a plurality of voxelsassociated with the three-dimensional model of the object in response tothe placement of the object within the three-dimensional scene, and inresponse to the three-dimensional model of the object, wherein eachvoxels from the plurality of voxels is associated with a subsurfacescattering parameter; determining an incident irradiance associated witha plurality of voxels associated with a surface region of the object;and determining a subsurface scattering contribution associated with theplurality of voxels associated with a surface region of the object inresponse to the incident irradiance and in response to the subsurfacescattering parameters associated with the voxels from the plurality ofvoxels.
 16. The method of claim 15 wherein the subsurface scatteringparameters associated with the voxels from the plurality of voxels areselected from a group consisting of: extinction coefficients, scatteringlengths.
 17. The method of claim 15 wherein determining the resolutionfor the volumetric grid comprises: determining a size for a voxel inresponse to the placement of the object within the three-dimensionalscene and the three-dimensional model of the object; determining thevolumetric grid comprising the plurality of voxels associated with thethree-dimensional model of the object in response to the size for thevoxel and in response to the three-dimensional model of the object. 18.The method of claim 15 wherein determining the subsurface scatteringcontribution comprising applying a recursive directional low-pass filteron the voxels from the plurality of voxels.
 19. The method of claim 15further comprising: displaying the image to the user; and storing arepresentation of the image on a tangible media.
 20. A computer programproduct for a computer system including a processor and a memorycomprising a computer-readable tangible media including executablecomputer code, the computer program product comprising: code configuredto direct the processor to receive a description of a three-dimensionalscene including a specification of placement of an object within thethree-dimensional scene; code configured to direct the processor toreceive a three-dimensional model of an object; code configured todirect the processor to determine a resolution for a volumetric gridcomprising a plurality of voxels associated with the three-dimensionalmodel of the object in response to the placement of the object withinthe three-dimensional scene, and in response to the three-dimensionalmodel of the object, wherein each voxels from the plurality of voxels isassociated with a subsurface scattering parameter; code configured todirect the processor to determine an incident irradiance associated witha plurality of voxels associated with a surface region of the object;and code configured to direct the processor to determine a subsurfacescattering contribution associated with the plurality of voxelsassociated with a surface region of the object in response to theincident irradiance and in response to the subsurface scatteringparameters associated with the voxels from the plurality of voxels.